A large number of infrared detection methods exist, each being based on a different working principal. Infrared detectors are being used in a large number of applications.
The present invention focuses on the group of detectors where the energy of the absorbed infrared radiation raises the temperature of the detecting element thereby changing its electrical conductivity. These detectors, known as bolometers, are fabricated from different materials like metals, permalloy, vanadium oxide, or (poly-crystalline) silicon.
In order to obtain a high performance, two points are important:    1) the total thermal conductance G from the resistor to the substrate must be low, so as to maximise the temperature increase for a given amount of energy deposited on the detector; and    2) the absolute value of the temperature coefficient of resistance (TCR) α (i.e., the percent variation of the device resistance for a temperature increase of 1 K) must be large.
The first point is related to the geometrical structure of the detector and to the thermal properties of the material(s) forming it, and the second one is related only to the electrical properties of the active material.
With technologies suggested in the state of the art of micro-machining good thermal insulation is obtained in two different ways either by micro-machining an electrically and thermally insulating membrane and depositing the active material onto it, either by micro-machining structures suspended over the substrate directly using the active material. This last approach is more simple and straightforward but requires an active material with low thermal conductance and with mechanical properties adequate for micro-machining. Until now, this is applied only to poly-crystalline silicon (poly-Si) bolometers.
An example of the first approach is reported in document WO-A-9313561 which describes a method for fabricating an integrated infrared sensitive bolometer having a polycrystalline element whereby an oxide region deposited on silicon nitride covered with a first polysilicon layer which is etched to provide a location for a bolometer element. A second polysilicon layer is deposited and doped to achieve a desired temperature coefficient of resistivity of 1 to 2%/° C. The second polysilicon layer forms an infrared sensitive element over the oxide region. Openings are etched in the infrared sensitive element to permit an etchant to remove the oxide region resulting in the sensitive element being suspended over the cavity. The thermal conductance is determined by the thermal conductivity of poly-Si and by the shape of the etch of the first poly-Si layer.
An example of the second approach is described in the document “Infrared Focal Plane Array Incorporating Silicon IC Process Compatible Bolometer” of Tanaka, et al. published in IEEE Transactions on Electron Devices, Vol. 43, No. 11, November 1996 which describes a 128×128 element bolometer infrared image sensor using thin film titanium. The device is a monolithically integrated structure with a titanium bolometer detector located over a CMOS circuit that reads out the bolometer's signals. By employing a metallic material like titanium and refining the CMOS readout circuit, it is possible to minimize 1/f noise. Since the fabrication process is silicon-process compatible, costs can be kept low.
The article “The Growth and Properties of Semiconductor Bolometers for Infrared Detection” of M. H. Uniwisse, et al in SPIE Vol. 2554/43 describes how to develop bolometer arrays from semiconductor materials, such as the amorphous and microcrystalline phases of Si, Ge, and SiGe. In this work, the use of amorphous and microcrystalline SiGe:H is suggested in order to reduce the large 1/f noise and the large resistivity of amorphous silicon. No use of the thermal properties of SiGe is mentioned.
The article “Thin Film Boron Doped Polycrystalline Silicon Germanium for the Thermopiles” of P. Van Gerwen, et al. in the 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25–29, 1995 describes the use of polycrystalline silicon-germanium for thermopiles instead of polysilicon. Thermopiles can be used for infrared detection if incident infrared is absorbed by an absorption layer near one junction which will heat up. The other junction is put on a heat sink, which will not heat. The temperature difference between the two junctions creates a voltage related to the absorbed infrared. These thermopiles are fabricated according to bulk micro-machining techniques.
Double-sided processing and special handling requirements though make bulk micro-machining incompatible with standard IC fabrication techniques.
U.S. Pat. No. 5,367,167 is describing a bolometer for detecting radiation in a spectral range including an integrated circuit substrate and a pixel body spaced from the substrate. The pixel body comprises an absorber material such as titanium for absorbing radiation in the spectral range. In addition, a variable resistor material which is the active element made of amorphous silicon is formed over an insulating layer.
The article “Thermal stability of Si/Si1-xGex/Si heterostructures deposited by very low pressure chemical vapor deposition” published in Applied Physics Letter, vol. 61, No. 3, of 20 Jul. 1992, pp; 315–316 describes structures using crystalline SiGe deposited on crystalline Si. More particularly, this document is related to the study of the thermal stability of metastable Si/Si1-xGex/Si strained structures deposited by very low pressure chemical vapor deposition.